In recent years, dental adhesive chemistry has seen much progress. It has advanced from a multi-component, multi-step process to a single-component, single-step process. In the past, there was a phosphoric acid etch step followed by a dental adhesive which infiltrated the dentinal tubules with a polymerizable resin carried into the dentinal tubules by a hydrophilic solvent such as ethanol or acetone. This adhesive layer was light polymerized, and a composite restoration was then placed on top of the adhesive and cured. This process resulted in a micro-mechanical attachment between the tooth and the restorative material. More recently, adhesive systems have been developed in which the phosphoric acid etch step has been eliminated. Phosphate-containing acid-monomers in an azeotropic solvent of ethanol and/or acetone with water have been used as a bond material. The acid monomers reduce the pH to a level suitable for etching and the azeotrope carries the polymerizable resin into the dentinal tubules. The material relies on micro-mechanical attachment, the association of like moieties for tooth integration, and the presence of phosphate ions to form a chemical bond to calcium ions in the tooth.
Calcium containing cements have been used as structural supports in orthopedic and dental applications. Recently, biologically active restorative materials were developed that may stimulate the repair of tooth structure through the release of cavity-fighting components including calcium and phosphate. The use of amorphous calcium phosphate (ACP) was disclosed as a bioactive filler encapsulated in a polymer binder. Calcium and phosphate ions released from ACP composites, especially in response to changes in the oral environment caused by bacterial plaque or acidic foods, can be deposited into the tooth structures as an apatite mineral, which is similar to the hydroxyapatite found naturally in teeth. The ACP has the properties of both a preventive and restorative material. This encourages its use in dental cements, sealants, composites, and, more recently, orthodontic adhesives. Though ACP has been used in some dental applications, its use in dental restorative materials is very limited due to its stability. In contact with water, ACP turns into hydroxyapatites. Another drawback of calcium phosphate cement is its low mechanical properties. Hydroxyapatite as a bulk solid does not have the necessary mechanical properties, such as strength or stiffness, to be used in load bearing applications. While much has been learned about the structure and growth of bone tissue due to modern microscopy, no reliable method of synthesizing this structure has been developed.